New and effective anticancer drugs need to be developed for patients that suffer from disseminated cancer. Developing drugs for solid tumours is associated with specific problems due to complex biophysical and metabolic conditions in 3-D tumour tissue which may be difficult to mimic in experimental in vitro systems. Hypoxia and limited diffusion of nutrients is known to lead to quiescence and resistance to conventional anticancer agents and radiation therapy. Furthermore, anticancer drugs must be able to penetrate into tumour parenchyme to reach cancer cells at toxic concentrations. Some drugs that are in clinical use for the treatment of solid tumours show poor penetration into 3-D tumour masses, which may be one of the reasons for their limited efficacy (1). Multicellular spheroids (MCS) mimic human solid tumours better than 2-D monolayer cultures (2-4), and many clinically used drugs show limited potency on cancer cells grown as MCS (5, 6). Therefore, MCS are better suited than monolayer cultures for screening drugs active on solid tumours.
Cell death is often subdivided into three types of cell death: apoptosis (type I), autophagic cell death (type II) and necrosis (type III). Apoptosis is mediated by the activation of caspases. Autophagy is an evolutionarily conserved mechanism for degradation of long-lived cellular proteins and damaged cell organelles. The formation of autophagosomes is a main characteristic of autophagy. Autophagosome formation requires activation of class III phosphatidylinositol-3-kinase and is also dependent of two ubiquitin-like conjugation systems (Atg-Atg12 and Atg8) (7). Autophagy protects cells during conditions of nutrient deprivation, and cells undergo apoptosis when autophagy is inhibited (8-10). Morphological features of autophagy have also been observed during cell death under conditions of caspase inhibition (11).